This invention relates to a heat pump device including metal hydrides.
It is known that a certain kind of metal or alloy exothermically occludes hydrogen to form a metal hydride, and the metal hydride endothermically releases hydrogen in a reversible manner. Many such metal hydrides have been known, and examples include lanthanum nickel hydride (LaNi.sub.5 H.sub.x), calcium nickel hydride (CaNi.sub.5 H.sub.x), misch metal nickel hydride (M.sub.m Ni.sub.5 H.sub.x), iron titanium hydride (FeTiH.sub.x), and magnesium nickel hydride (Mg.sub.2 NiH.sub.x). In recent years, heat pump devices built by utilizing the characteristics of the metal hydrides have been suggested (see, for example, Japanese Laid-Open Patent Publication No. 22151/1976).
One example of such conventional heat pump devices comprises a first receptacle having filled therein a first metal hydride, a second receptacle having filled therein a second metal hydride, the first and second metal hydrides having different equilibrium dissociation characteristics, a hydrogen flow pipe connecting these receptacles in communication with each other, and heat exchangers provided in the respective receptacles. According to this heat pump device, a heating output and a cooling output based on the heat generation and absorption of the metal hydrides within the receptacle are taken out by means of a heat medium flowing within the heat exchangers. This type of heat pump is called an internal heat exchanging-type heat pump. The receptacles of the conventional heat pump should withstand the pressure generated at the time of hydrogen releasing of the metal hydrides and the total weight of the filled metal hydrides and the heat exchangers. Accordingly, the receptacles have a large wall thickness and a large weight, and become complex in structure.
Furthermore, since in the conventional metal hydride heat pumps, a metal hydride in an amount required per unit time is wholly filled in each receptacle, the reaction of the metal hydride in the receptacle is exceedingly non-uniform, and the loss of heat by radiation from the joint parts of the receptacles including the hydrogen flow pipe, and the loss of heat owing to heat transmission attributed to the temperature difference between the receptacles, markedly reduce the coefficient of performance of the heat pump devices.
According to another conventional practice, two heat pumps of the above structure are provided in juxtaposition and operated with a phase deviation of a half cycle, whereby a cooling output and a heating output can be obtained alternately, and therefore continuously as a whole, from the respective heat pumps.
One example of such a conventional device is shown in FIG. 1. The operating cycle of the device of FIG. 1 for obtaining a cooling output is shown in FIG. 2. FIG. 3 is a temperature distribution chart within a heat exchanger during the operation of the device of FIG. 1.
The device of FIG. 1 is built by filling a first metal hydride M.sub.1 H and a second metal hydride M.sub.2 H having different equilibrium dissociation characteristics in a first closed receptacle 1 and a second closed receptacle 2 and connecting the two receptacles by a communicating pipe 6 having a valve 5, and similarly connecting closed receptacles 3 and 4 containing M.sub.1 H and M.sub.2 H respectively by means of a communicating pipe 7. When this device is to be operated to obtain a cooling output, M.sub.1 H in the first receptacle 1 [to be abbreviated (M.sub.1 H).sub.1 ] is heated to a temperature T.sub.H by means of a heat exchanger 8 disposed within the receptacle 1 thereby to release hydrogen (point A in FIG. 2). The released hydrogen is sent to the second receptacle 2 through the communicating pipe 6 where M.sub.2 H in the second receptacle 2 [to be abbreviated (M.sub.2 H).sub.2 ] exothermically occludes hydrogen (point B in FIG. 2) while being cooled to a temperature T.sub.M by means of a heat exchanger disposed within the increases from the heat medium inlet toward the outlet of the receptacle 2. Consequently, M.sub.2 H existing in the downstream portion of the heat exchanger 9 attains a temperature T.sub.M', which is higher than the temperature T.sub.M. In this way, the difference in temperature, i.e. the difference in equilibrium dissociation pressure, between the metal hydrides in the downstream portion of the heat-exchanger decreases, and the rate of hydrogen transfer from point A to point B decreases. In some cases, hydrogen transfer might stop locally. This means that the output per unit time is low. In particular, since in a conventional metal hydride heat pump, a metal hydride in an amount which can give the required output per unit time is wholly filled in each receptacle, the reaction of the metal hydride within the receptacle becomes exceedingly non-uniform.
The non-uniformity of the reaction also occurs when hydrogen is transferred from point D to point C in FIG. 2.